In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such hot process gas containing, among other components, mercury, Hg. Since mercury is hazardous to the human health and to the environment it is usually necessary to remove mercury from the process gas before it can be discharged into the ambient air. The removal of mercury is often accomplished by mixing the hot process gas with a solid sorbent, such as pulverized activated carbon, which adsorbs the mercury and which can then be removed from the hot process gas in a filter, such as a bag house.
U.S. Pat. No. 5,505,766 describes a gas cleaning system in which a sorbent is supplied to a bag house from a silo. The sorbent forms a sorbent layer on filter bags of the bag house. When a sufficiently thick sorbent layer has been formed on the filter bags, the hot process gas is admitted to the bag house. When the removal of mercury decreases a compartment of the bag house is taken off-line, such that the mercury loaded sorbent can be removed, and new sorbent can be added to the bags, before the process gas is again allowed to enter the compartment in question.
While the gas cleaning system of U.S. Pat. No. 5,505,766 ensures that a layer of sorbent will always be available on the filter bags for the removal of mercury from the process gas, it is also a complicated process that requires an advanced control and expensive equipment. Furthermore, the bag house must be over-sized compared to normal operation, to allow that one compartment at a time is taken off-line for cleaning.